Semiconductor device and method for fabricating the same

ABSTRACT

The present invention relates to the field of the semiconductor fabrication. Also, the objects of the present invention are to provide a semiconductor device and method for fabricating the same having the MOS transistors capable of improving the thermal conduction characteristics and the punch-through and the DIBL effect. To accomplish these objects, the present invention provides the semiconductor device including a semiconductor substrate; a first insulating layer, a selected material layer and a second insulating layer orderly stacked on said semiconductor substrate; and a semiconductor layer formed on the second insulating layer for providing an active area where MOS transistors are formed, wherein, said material layer provides a path for emitting heat generated from said MOS transistors.

This application is a divisional application of U.S. application Ser. No. 09/375,359, filed on Aug. 17 1999 and now U.S. 6,383,849; which is a continuation-in-part application of Ser. No. 08/886,155, filed on Jun. 30, 1997; now abandoned, and claims priority from Korean Application No. 1996-26525, filed Jun. 29, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of the semiconductor fabrication, and more specifically to a MOS transistor for fabricating a DRAM device.

2. Description of Related Art

In general, to increase the integration of semiconductor devices, the size of each of the elements in the semiconductor devices must be reduced. In particular, to reduce the size of a MOS transistor, the adding impurity concentration of the channel region in the MOS transistor must be increased, thereby leading a higher parasitic capacitance of the MOS transistor. Accordingly, the electric characteristics of the MOS transistor are degraded with reduction of the size of the MOS transistor, so it is required that a low operating voltage is applied to the MOS transistor to maintain the expected life span.

As described above, with the development of highly integrated circuits, there are various problems to be solved. In particular, there are many problems awaiting solution in increase of a electric power consumption and in decrease of a operating speed. And also, the DIBL(Drain Induced Barrier Lowering) effect causing the electric field generated from the drain region to have influence upon the source region due to the shortage of the channel length and the punch-through effect may increase the leakage current of the MOS transistor.

To solve the above problems, a SOI (Silicon On Insulator) structure is proposed as shown in FIG. 1. In this case, an insulating layer 12 on a substrate 11, which is called a buried oxide, is formed with silicon oxide, and then parasitic capacitance is reduced by the buried oxide. Accordingly, the operating speed of circuits are increased. However, it is difficult to transfer downward the heat occurred in the MOS transistors, because the thermal conductivity of silicon oxide is {fraction (1/100)} times as low as silicon.

Therefore, it is a factor that decreasing the efficiency of the device deteriorates with the increase of the heat in the MOS transistor. And also, the leakage current of the MOS transistor may be greatly increased, because the electric field generated from a drain 14 propagates to the buried oxide layer in the MOS transistor. These characteristics become worse with smaller size of the MOS transistor.

In FIG. 1, reference numeral 11 denotes a silicon wafer, 13 a source, 15 a channel region, 16 a gate oxide layer, 17 a gate electrode.

The ideal structure of the MOS transistor may be implemented, by forming three impurity regions different from one another in the channel beneath the gate oxide layer. That is, a low impurity concentration layer doped, for example, less than 1×10¹⁶ ions/cm³ is formed up to a predetermined depth, for example, 300 Å from the gate oxide layer to increase mobility of electrons or holes and to increase the driving capacity of the MOS transistor. Subsequently, beneath the low impurity concentration layer, a high impurity concentration layer doped, for example, more than 1×10¹⁷ ions/cm³ is abruptly formed up to reduce the punch-through effect and the DIBL effect from being generated in the MOS transistor and, at a predetermined depth, for example, 1000 Å a low impurity concentration is maintained again to decrease the parasitic capacitance of the source/drain regions. However, it is difficult to embody such a MOS transistor because of the impurity diffusion at a high temperature required in forming an gate oxide layer. That is, although a lay-out for forming such a ideal MOS transistor is provided, the impurities in the higher impurity concentration layer are diffused into the lower impurity concentration layers, at the time of growing the gate oxide layer at a high temperature. As a result, parasitic capacitance is increased by the downward diffused impurities, the current driving capacity is decreased by upward diffused impurities.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a semiconductor device and method for fabricating the same having the MOS transistors capable of improving the thermal conduction characteristics.

Also, another object of the present invention is to provide a semiconductor device and method for fabricating the same having the MOS transistors capable of improving the punch-through and the DIBL effect caused by the electric field generated from the drain region of the MOS transistor.

In accordance with an aspect of the present invention, there is provided a semiconductor device comprising: a semiconductor substrate; a first insulating layer, a selected material layer and a second insulating layer orderly stacked on said semiconductor substrate; and a semiconductor layer formed on the second insulating layer for providing an active area where MOS transistors are formed, wherein, said material layer provides a path for emitting heat generated from said MOS transistors.

In accordance with another aspect of the present invention, there is provided method of fabricating a semiconductor device comprising the steps of: forming a first insulating layer on a first semiconductor substrate; forming a polysilicon layer on the first insulating layer; forming a second insulating layer on the polysilicon layer; adhering a second semiconductor substrate to the second insulating layer; leaving a part of one of the first and second semiconductor substrate by using chemical mechanical polishing recipe; forming a gate insulating layer and a gate electrode on the whole structure; and forming junctions by using implantation to a selected depth.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional view illustrating a conventional MOS transistor with an SOI structure;

FIGS. 2A-2E is a cross-sectional view illustrating a method for fabricating a MOS transistor in accordance with an embodiment of the present invention; and

FIG. 3 is a cross-sectional view illustrating a MOS transistor in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will be described detail referring to the accompanying drawings.

As shown in FIG. 2A, a thin oxide layer 22 a is formed on a silicon wafer 21 to a thickness of 50 Å to 150 Å. At this time, a thermal oxidation or a deposition process may be carried out for the forming of the oxide layer 22 a.

Next, as shown in FIG. 2B, after depositing a polysilicon layer 28 on the oxide layer 22 a, a oxide layer 22 b is formed on the polysilicon layer 28 to a thickness of 50 Å to 150 Å, by applying the thermal oxidation or the deposition process. A polysilicon is much better than a silicon oxide in the thermal conductivity.

Next, FIG. 2C shows that, after forming a very thin oxide layer (not shown) to improve adhesion on another silicon wafer 25, joined with the oxide layer 22 a which is formed on the polysilicon layer 28.

Next, as shown in FIG. 2D, leaving a part of the silicon wafer 21 which is enough need to form junctions, CMP (Chemical Mechanical Polishing) process is applied to a top portion of the silicon wafer 21.

Next, as shown in FIG. 2E, after forming a gate oxide layer 26 and a gate electrode 27 on the resulting structure in due order, a source region 23 and a drain region 24 are formed by the impurity-implantation method.

In an embodiment of the present invention as above, instead of the berried oxide in conventional SOI structure, the polysilicon layer 28 and the oxide layers 22 a and 22 b are used.

In this structure, the polysilicon layer 28 is doped with a impurity concentration less than 1×10¹⁶ icons/cm³ when a V_(TH) implantation for the control of the threshold voltage of the MOS transistors is carried out.

In This MOS transistor according to the present invention, the multi-layer including the oxide layer 22 a, the polysilicon layer 28 and the oxide layer 22 b may not only reduce the parasitic capacitance of the source region 23 and drain region 24 as the buried oxide layer of the conventional SOI structure, but also transfer the heat, which is occurred in the MOS transistor, to the silicon wafer 25.

FIG. 3 is a cross-sectional view illustrating a MOS transistor in accordance with another embodiment of the present invention. The thickness of silicon wafer 21 is formed far thinner than that as shown in FIG. 2E, and its lower layers, the multi-layer that of a oxide layer 22 a, a polysilicon layer 28 and a oxide layer 22 b, is also formed to a very thin thickness. At this time, a source region 23 and a drain region 24 is defined on the oxide layer 22 a, and the impurity doped layer located under the source region 23 and the drain region 24 can be formed inside of the polysilicon layer 28.

At this time, the impurity doping concentration of the impurity doped layer in the polysilicon layer 28 is more than 1×10¹⁷ ions/cm³.

Such a impurity diffused layer may reduce the punch-through or DIBL effect effectively, because there exist the doped polysilicon layer 28 in the path where the electric field generated from the drain region has an affect on the source region 23. It is possible to keep the concentration of a silicon layer, in which active regions are formed, low.

In this case, although it goes via the high thermal process in MOS transistor fabricating process, the oxide layers 22 a and 22 b may reduce the diffusion of impurities that is in the polysilicon layer 28.

If it may not reduce the impurity diffusion with the oxide layers 22 a and 22 b because they are very thin, it may use a ONO (Oxide-Nitride-Oxide) layer instead of the oxide layers 22 a and 22 b.

In the embodiments of the present invention, a silicon layer etc., which are profitable to the thermal conduction, instead of the polysilicon layer 28.

As apparent from above, the present invention improves the thermal conduction characteristics of the MOS transistors, and reduces the extension of the electric field generated from the drain to the silicon substrate. Therefore, the punch-through and DIBL effects can be greatly reduced.

Also, it may reduce the parasitic capacitance, when the impurity concentration of the polysilicon layer is kept low, by which the polysilicon layer is depleted and operates like a insulating layer. Therefore, the power consumption of the semiconductor device may be reduced and the operating speed of it may be improved.

Although specific embodiments of the present invention are herein described, they are not to be constructed as limiting the scope of the invention. Many embodiments of the present invention will become clear to those skilled in the art in light of the methodology of the specification. The scope of the invention is limited only by the claims appended. 

What is claimed is:
 1. A semiconductor device comprising: a semiconductor substrate; a first insulating layer, a polysilicon layer and a second insulating layer orderly stacked on said semiconductor substrate wherein an impurity doped layer is formed with an impurity concentration of more than 1×10¹⁷ ions/cm³ inside of the polysilicon layer to reduce a punch-through or DIBL effect; a semiconductor layer formed on the second insulating layer for providing an active area where MOS transistors are formed; and a gate and a gate oxide film formed on the semiconductor layer, wherein, said polysilicon layer provides a path for emitting heat generated from said MOS transistors.
 2. A semiconductor device according to claim 1, wherein the first and second insulating layers are oxide layers.
 3. A semiconductor device according to claim 1, wherein the first and second insulating layers are oxide-nitride-oxide layers.
 4. A semiconductor device according to claim 2, wherein said oxide layers have the thickness of 50 Å to 150 Å.
 5. A semiconductor device according to claim 1, wherein the impurity doped layer operates as a barrier of the electric field generated from the junctions. 